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(1) Field of the Invention
The present invention relates to a process for the separation of a polyol from an aqueous solution. The process involves reactive distillation of the polyol as a cyclic acetal from an aqueous reaction mixture containing other organic compounds, particularly other polyols. In particular, the cyclic acetal is prepared by reaction of a ketone or aldehyde with the polyol along with distillation of cyclic acetal as it is formed from the reaction mixture.
(2) Description of the Related Art
There is a need to recover and purify polyols, including glycols, from an aqueous solution. These polyhydroxy compounds are typically formed in multistep processes in dilute aqueous solutions, from which the polyol(s) must be separated and purified before being used or sold. These processes include, but are not limited to, production of ethylene glycol and propylene glycol from their respective epoxides, formation of propylene glycol from glycerol, and formation of polyols via hydrogenolysis of sugars or sugar alcohols. All of these processes produce dilute mixtures of organic compounds including the desired polyols in the aqueous solution.
In the presence of acidic catalysts, glycols (or other polyols) react reversibly with aldehydes and ketones to form cyclic acetals. The reaction is known as acetalization or ketalization. The acetals of the polyols are far more volatile than the polyols themselves and much less polar, making them easily separable from water by distillation. Because the acetalization reaction is reversible, glycols and the aldehyde can be regenerated by acid hydrolysis of the acetal. The glycol can then be recovered and the aldehyde can be recycled. Ion exchange resins (IER) are one class of materials that can effectively catalyze acetal formation and hydrolysis, but mineral acids and other solid acids are effective as well. The reaction is as follows: 
Of general interest in connection with this type of reaction in a non-cyclic context is Mahajani, S. M. et al., Reactive And Functional Polymers 28 29-38 (1995).
There have been several reports of the reaction of glycols with aldehydes to form cyclic acetals. Tink and coworkers (Tink, R. R., et al., Can. J. Technol., 29, 243 (1951)) have published a series of papers describing recovery of aqueous glycerol solution via reactive extraction with various aldehydes. As disclosed, n-butyraldehyde and cyclohexanone were promising among the several aldehydes studied and the former was particularly selective. They also studied reactive extraction of several polyhydroxy compounds including D-sorbitol, adonitol, dulcitol, D-mannitol and ethylene glycol from aqueous solutions. High distribution coefficients were obtained with reactive extraction. For instance, with n-butyraldehyde the distribution coefficient for glycerol is 8.3, for EG is 5.9 and for D-sorbitol is 788.
Broekhuis et al. (Broekhuis, R. R., et al., Ind. Eng. Chem. Res., 33, 3230 (1994)) have compared the various routes for the recovery of propylene glycol from dilute aqueous solutions via reaction with aldehydes. They studied lower aldehydes, viz. formaldehyde and acetaldehyde, for reactive distillation and extractive reaction for the recovery. They have claimed to achieve 99+% recovery of propylene glycol in a reactive distillation process. One of the present inventors has reported on the recovery of ethylene glycol from aqueous solution via acetalization with formaldehyde (Chopade, S. P. and Sharma, M. M., React Funct. Polym. 34(1) 37 (1997)) using ion exchange resins as catalysts.
A search of the patent literature reveals no processes combining acetalization with reactive distillation of cyclic acetals for polyol separation. U.S. Pat. No. 5,917,059 to Bruchmann et al. describes preparation of the cyclic acetal compounds, but does not discuss them in context of a separation scheme for glycol recovery. There are numerous patents that describe inventions pertaining to acetals, acetalization, and reactive distillation, but none were found that pertained to a scheme for the recovery of polyols, especially from a dilute mixed solution of polyols, such as a sugar hydrogenolysis effluent.
Polyhydroxy compounds show a high affinity towards water and each other because of hydrogen bonding, and separation of these products from aqueous solution is conventionally done via a multi-column distillation process. In order to obtain ethylene glycol (EG) and propylene glycol (PG), water must be distilled off first because it has a lower boiling point temperature than the polyols. The energy to distill off water is the primary reason for the high cost of polyol separation and recovery. Separation of EG and PG (if they are present together) is also costly because they have very similar boiling points, so that a large number of stages and a large reflux ratio, translating to a large distillation column, is required to achieve the required purities. Purification of glycerol in a simple distillation column without forming poly-glycerides and decomposition products is impossible. Vacuum distillation, which has high operating costs, is the only distillation route for direct glycerol recovery.
Another approach for polyol recovery is solvent extraction of polyols from water. Glycols and glycerol have high affinity towards water (again because of hydrogen bonding), and it is difficult to find a suitable solvent with good distribution coefficient and low miscibility with water. Further, extraction only eliminates distillation of large amounts of water from the product stream. After extraction, there are distillation steps involving solvent recovery followed by separation of polyols from each other. Thus extraction is similar to distillation, except that water is replaced by a solvent.
There is a need for a safe and effective process for the production of polyols. In particular there is a need for a process to efficiently separate EG and PG from aqueous solutions.
It is therefore an object of the present invention to provide an economical and efficient process for the separation of at least one polyol from water. It is further an object of the present invention to provide a process which is relatively easy to perform on a large scale suitable for commercial production of polyols such as EG and PG. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to a continuous process for preparing at least one acetal from an aqueous solution containing at least one polyol and at least one other organic compound which comprises:
(a) reacting in a combination reaction and distillation vessel a reaction mixture of the polyol and an aldehyde or ketone containing 1 to 4 carbon atoms in the aqueous solution in the presence of an acid catalyst, wherein the reaction mixture is introduced into the reaction vessel containing the catalyst with a molar excess of the aldehyde or ketone over the polyol to produce the cyclic acetal in the aqueous solution; and
(b) separating at least one cyclic acetal from the reaction mixture by distillation.
Further, the present invention relates to a continuous process for recovering a polyol from an aqueous solution containing other organic compounds which comprises:
(a) reacting in a combination reaction and distillation vessel a reaction mixture of the polyol and an aldehyde or ketone containing 1 to 4 carbon atoms in the aqueous solution, wherein the reaction mixture is continuously introduced into the vessel containing the catalyst with a molar excess of the aldehyde or ketone over the polyol to produce a cyclic acetal in the aqueous solution;
(b) separating the acetal from the mixture at elevated temperatures; and
(c) hydrolyzing the cyclic acetal produced to recover the polyol as a liquid and the acetaldehyde or ketone which is separated as a vapor from the polyol.
Preferably the reaction mixture is at a temperature, less than the boiling point of the reaction mixture, at which at least the aldehyde or ketone is distilled from the reaction vessel as a distillate. Also, preferably if there is more than one cyclic acetal produced, the cyclic acetals are separated before the hydrolysis step is performed. The separation can be accomplished in the reaction vessel for the reactive distillation or in a separate vessel connected to the reaction vessel. Typically the reaction vessel(s) is a heated column. Preferably the desired cyclic acetal is also distilled from the reaction vessel and separated from the aldehyde or ketone.
Preferably the reaction mixture is at a temperature, less than the boiling point of the reaction mixture, at which at least the aldehyde or ketone is distilled from the aqueous solution as a distillate. Preferably the ketone or aldehyde is recycled.